Cascode power amplifier

ABSTRACT

An amplifier for amplifying signals is presented. A cascode power amplifies includes two or more adjacent cascode amplifiers and at least one remote cascode amplifier. The adjacent cascode amplifiers are lined up adjacent each other with inputs of the adjacent cascode amplifiers connected to a common input line and outputs of the of adjacent cascode amplifiers connected to a common output line. The adjacent cascode amplifiers generally operate in parallel. The remote cascode amplifier is spaced apart from the adjacent cascade amplifiers. An input transmission line connects an input of the remote cascode amplifier to the common input line. An output transmission line connects an output of the remote cascode amplifier to the common output line. Amplified outputs of the adjacent cascode amplifiers and amplified outputs of the remote cascode amplifier are power combined and summed into a coherent amplified output signal that is output on the output transmission line.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional application of U.S. Ser. No. 14/028,844 filed Sep.17, 2013 and claims priority from U.S. Provisional Application Ser. No.61/701,888, filed Sep. 17, 2012; the disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

The current invention relates generally to apparatus, systems andmethods for amplifying a signal. More particularly, the apparatus,systems and methods relate to amplifying a radio frequency (RF) powersignal. Specifically, the apparatus, systems and methods provide for apower amplifier that uses multiple cascode amplifiers some of which aregrouped together and some of which are spaced apart.

2. Description of Related Art

Amplifiers have long been used to amplify a variety of electricalsignals. For example, amplifiers can be used to amplify voltage,current, power and the like. Early amplifiers used vacuum tubes toamplify signals. These tubes where large, used high power and oftenburned out. The invention of the silicon transistor greatly improvedamplifier technology and quickly led to the extinction of vacuum tubes.Silicon transistors were much smaller, cheaper, could be more easilymass produced and had a much longer life span than vacuum tubes.Additionally, transistors consume much less power and generate less heatthan vacuum tubes.

Because of a transistors small size, it has allowed for moresophisticated amplifiers to be designed. For example, operationalamplifiers (Op Amps) contain two or more stages of amplification eachwith their own bias schemes all implemented with transistors and otherdiscrete components. Op Amps provide excellent common mode rejection sothat just a signal of interest is amplified.

One conventional approach to amplifying radio frequencies (RF) is to usea cascade amplifier that has a common gate transistor and a commonsource transistor. However, these types of amplifiers often have a smalloperational bandwidth (BW) and cannot handle higher currents/power,Therefore, what is needed is a better amplifier.

SUMMARY

The preferred embodiment includes a cascade power amplifier (PA). Thecascode PA is an RE power amplifier (PA) that includes two or moreadjacent cascode amplifiers and at least one remote cascode amplifier.The adjacent cascode amplifiers are lined up adjacent each other withinputs of the adjacent cascade amplifiers connected to a common inputline and outputs of the of adjacent cascode amplifiers connected to acommon output line. The adjacent cascade amplifiers generally operate inparallel. The remote cascade amplifier is spaced apart from the adjacentcascode amplifiers. An input transmission line connects an input of theremote cascade amplifier to the input line and to the common input line.An output transmission line connects an output of the remote cascodeamplifier to the output line and the common output line. Amplifiedoutputs of the adjacent cascade amplifiers and amplified outputs of theremote cascade amplifier are all power combined and summed into acoherent amplified output signal that is output on the outputtransmission line.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

One or more preferred embodiments that illustrate the best mode(s) areset forth in the drawings and in the following description. The appendedclaims particularly and distinctly point out and set forth theinvention.

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate various example methods, and otherexample embodiments of various aspects of the invention. It will beappreciated that the illustrated element boundaries (e.g., boxes, groupsof boxes, or other shapes) in the figures represent one example of theboundaries. One of ordinary skin in the art will appreciate that in someexamples one element may be designed as multiple elements or thatmultiple elements may be designed as one element. In some examples, anelement shown as an internal component of another element may beimplemented as an external component and vice versa. Furthermore,elements may not be drawn to scale.

FIG. 1 illustrates an example schematic of a preferred embodiment of acascode radio frequency (RF) power amplifier (PA).

FIG. 2 illustrates an example view looking downward toward a galliumnitride (GaN) chip implementing a Non-uniform Distributed PA (NDPA).

FIG. 3 illustrates an example top view a preferred embodiment of a biasinductor formed in a metal layer and with air bridge connector devices.

FIG. 4 illustrates an example cross-section view a preferred embodimentan air bridge.

FIG. 5 illustrates an example top view of the metal layers of a FRAPcircuit.

Similar numbers refer to similar parts throughout the drawings.

DETAILED DESCRIPTION

FIG. 1 illustrates the preferred embodiment of a cascode Power Amplifier(PA) cell 100 that uses a compound transistor. The compound transistorsinclude a common source transistor X1 and a common gate transistor X2.They are connected in series from a DC standpoint but in cascodeconfiguration from an RF standpoint. The advantages to this compoundtransistor over a conventional single ended common source transistor isthat first, it has a high efficiency. Secondly, it a higher voltage andlower current for a given power output which reduces certain powerdistribution loses both in the power module and in the chip itself dueto reduced Ohmic losses operating at higher voltage and lower current.As a result of the higher voltage and lower current, a given powerimpedances are higher so that they can be matched over a wider bandwidth(BW).

The novelty of this embodiment of the PA cell 100 includes the biasnetwork and how it stabilizes the cascade PA cell 100. The two left-handbiasing legs of FIG. 1 are the RF cascading and stabliization circuits.These two legs include R1-R3, C1, TL1, TL6 and TL7. There is a resistivevoltage divider formed with resistors R1 and R2 connected to the commongate transistor X2 through a transmission line that sets the voltage ofthe compound transistor to half of Vdd across the common gate transistorX2 and half of Vdd across the common source transistor X1. There also isa series RC formed by resistor R3 and capacitor C1 combination thatallows cascading grounding of the common gate of transistor X2, that isessentially an RE ground. Ideally, a large capacitor C1 is desired butthis would require too much area and a small cascode cell is desired.Therefore, in the preferred embodiment C1 is still made as large aspossible within a confined area and R3 is connected in series with it.

The common gate transistor X2 makes a very good oscillator configurationso stability can be controlled. The common gate transistor X2 has itssource connected to the drain of the common source transistor X1 and itsdrain connected to the output P1 and resistor R1. Common sourcetransistor X1 has its gate connected to RF ground (capacitor C2). Thecommon transistor X1 has a gate connected to an input line and has itsdrain connected to resistor R2 and capacitors C1, C2 and has its sourceconnected to ground.

In the preferred embodiment, the value of the components in FIG. 1 arenow provided. R1 and R2 are each 5000 Ohms and have widths of 10 micrometers (um) and lengths of 123 um. Resistor R3 has a value of 320 Ohms,a width of 12.5 um and a length of 10 um. Capacitor C1 has a value of1.0 pF and capacitor C2 has a value of 0.085 pF. Transmission line TL1has a width of 8 um and a length of 155 um, transmission line 112 has awidth of 15 um and a length of 105 um, transmission line TL3 has a widthof 10 um and a length of 40 um, transmission line TL4 has a width of 8um and a length of 185 um, transmission line TL5 has a width of 8 um anda length of 58 um. Transmission line T6 has a width W1 of 14, a with W2of 14 um and a width W3 12 um, transmission line 17 has a width W1 of 10um, a width W2 of 10 um, a W3 18 um and transmission line T8 has a withW1 of 14 um, a width W2 of 14 um and a width W3 12 um.

FIG. 2 illustrates the preferred embodiment of how some components andcells are positioned and laid out on a Salt chip to create a RF PA. Thechip can he implemented with GaN or with another type of semiconductormaterial as understood by one of ordinary skin in the art. FIG. 2,illustrates both halves 3A, 3B of cascaded RF PA 1 that is symmetricalabout centerline CL1 that cuts the RF PA 1 into two halves 3A, 3B.Because it has a lot of symmetry, only one half 3A will be described andthat description and labeling will equally apply to the second half 3B.The PA 1 is a non-uniform distributed PA for two reasons, First thewidths of the transmission lines are different resulting in differentimpedances. Secondly, it is non-uniform because the cascode cells 100are distributed with a duster eight cascode cells 5 (e.g., eightamplifiers 100) clumped together at one location and with two othercascode cells 7, 9 distributed remotely away from the cluster of eight5.

The RF input enters the Non-uniform Distributed PA (NDPA) transmissionline TL10 before passing by capacitor C1 and onto a tapered transmissionline TLT connected to the bank of eight cascaded cells 5 (e.g., eighthamplifiers 100). Transmission line TLT is generally tapered so that itbecomes smaller in width until the last cascode amplifier 100 of thecluster of eight cascaded cells 5 receives the RF input signal.

Transmission line TL11 is formed with transmission lines TL11A andTL11B. Transmission line TL11A is connected to the end of the taperedtransmission line TLT and is also connected to the remote cascadeamplifier 7. Transmission line TL11A includes a generally semicircleportion 21 that is included to increase the length of transmission lineTL11 to make it a proper length. Transmission line TL11B is connectedbetween remote cascade amplifier 7 and remote cascade amplifier 9.Transmission line TL11B is straight between remote cascade amplifier 7and remote cascade amplifier 9 and has a constant width between thesetwo amplifiers.

Transmission line 13 is formed with transmission lines TL13A-C.Transmission line 13A is connected to the outputs of the cluster ofeight cascaded cells 5. This transmission line TL13A is slightly taperedbeginning at the first cascade amplifier 100 of the bank of eightcascaded cells 5 until it reaches the last cell 100 of the bank ofcascaded cells 5. Transmission line TL13B is connected to transmissionline TL13A at the last cell 100 of the bank of cascaded cells 5 andtransmission line TL13B is routed from here to the output of remotecascade amplifier 7. This transmission line TL13B is jogged way fromtransmission line 11A for shielding reasons. Transmission line 13C isconnected between the outputs of remote cascade amplifier 7 and remotecascade amplifier 9. This transmission line 13C is straight with aconstant width.

Output transmission line TL14 is connected between the output of remotecascade amplifier 9 and an output capacitor C6. it is also connected toa biasing inductor I1. This transmission line TL14 includes a somewhatsemicircular portion 23 that extends its length a desired amount foroptimal operation. Bias inductor I1 is connected/wired to capacitors C2and C3. The mirrored cacsode RF PA 1 contains other capacitors C4, C5and other components that are not discussed in detail here as they arenot the primary novelty of the preferred embodiment of the cascade RF PA1.

FIGS. 3 and 4 illustrated the bias inductor I1. The bias inductor I1 hastwo levels of metal. One layer of metal is a transmission strip 25 layerof metal in combination with a spiraling octagonal shape metal 31 andthe another layer of metal includes metallic air bridge metal structures27 that air bridge over the transmission strip of 25 metal passing underthe air bridge metal 27. There is actually a gap 41 between the airbridge metal 27 and the transmission strip 25. This gap can be filledwith air, another gas or another material as understood by those ofordinary skill in the art. The air bridge metal 27 can include tab ends29A, 29B that are used to connect it to ends 31A, 31B of the spiraledmetal 31. The air bridge metal 27 actually arches upward from the firstend 31A of the spiral metal 31 and has a curved arch that later curvesdownward toward the second end 31B of the spiral metal 31. A centralportion 33 of the spiral of the bias inductor I1 is free of metal. Inthe preferred embodiment, the spiraled metal 31 almost makes fivecomplete spirals around the central portion 33. Of course, in otherembodiments, a different number of completed spirals may be desired.

It is desired to have an RF PA that has high bandwidth which means thatthe bias inductor I1 ideally has high impedances that don't interferewith the desired RE signal. Thus a large inductance is preferred, but alarge inductance has a parasitic that is a shunt capacitance that limitsthe BW. However, the bias inductor of FIGS. 3 and 4 has an overall goodgeometry that does well to balance these competing design constraints.The conductors are thick and wide enough to handle the high current ofthe PA 1. In the preferred embodiment, the width (W) of the metal 31used to form the octagonal shaped inductor I1 is about 40 microns widewith about 10 microns of gap (G) between the metal spirals. Of coursethese measurements can be other values.

FIG. 5 illustrates the details of the fusible link resistive voltagedivider “FRAP” device 70. Before the invention of this FRAP 70 oneneeded to apply a gate voltage to each individual chip and eachindividual chip needed to be tracked and the proper voltage applied topower it when it was implemented in a circuit. The FRAP device 70 isused to adjust the bias point of biasing circuits at the time of wafertesting. In the preferred embodiment, the FRAP 70 is on a GaN wafer 71with conductive electrical routing and pad components. Five resistorsR1-5 are provided and are connected to pad devices 77 that are connectedto fusible links 73. In the preferred embodiment, these five resistorscan be used to create about 32 different voltages ranging from −9 voltsto about −2 volts but other ranges and voltage could be created in otherembodiments. Two more resistors R6-7 are also provided that are alwaysused to create a bias voltage. Resistor R7 is connected by a pad 79 to areference voltage, that in the preferred embodiment is −9 volts.Resistor R6 is connected to the other ends of the fusible links by a padat a ground voltage and conductive routing 75. In the preferredembodiment, the values of the resistors is as follows: R1=75 ohms,R2=150 ohms, R3=300 ohms, R4=600 ohms, R5=1200 ohms, R6=75 ohms andR7=80 ohms. Of course, in other embodiments the resistors can havedifferent values and there may be fewer or more resistors used toimplement the FRAP 70.

At the time of wafer testing, the bias voltage of the RF PA 1 ismeasured while it being applied to the RF PA circuits themselves. Next,a determination is made as to how much the bias voltage needs to hechanged so that the RF PA 1 is biased to a proper value. A calculationis performed to determine which of the five resistors R1-5 connected tothe fusible links 73 need to be used to create the proper bias voltage.Once that is determined, the fusible links 73 connected to just theunneeded resistors are broken so that just the required resistorsparticipate in creating the proper bias voltage. In the preferredembodiment, the FRAP is a voltage divider circuit formed by resistersR1-R5. The fusible links 73 can be broken on the GaN wafer 71 by anymethod as understood by those of ordinary skill in the art. For example,one way they can be broken is applying a strong enough voltage acrossthem to create the breakage.

The related and co-owned U.S. Applications entitled “TILE ARRAY PAMODULE USING QUADRATURE BALANCED PA MIMICS,” “DIGITALLY CONTROLLED POWERAMPLIFIER,” and “METHOD OF OPERATING A POWER AMPLIFIER IN CLASSF/INVERSE CLASS F,” which are filed contemporaneously herewith, areincorporated as if fully rewritten.

In the foregoing description, certain terms have been used for brevity,clearness, and understanding. No unnecessary limitations are to beimplied therefrom beyond the requirement of the prior art because suchterms are used for descriptive purposes and are intended to be broadlyconstrued. Therefore, the invention is not limited to the specificdetails, the representative embodiments, and illustrative examples shownand described. Thus, this application is intended to embracealterations, modifications, and variations that fall within the scope ofthe appended claims.

Moreover, the description and illustration of the invention is anexample and the invention is not limited to the exact details shown ordescribed. References to “the preferred embodiment”, “an embodiment”,“one example”, “an example”, and so on, indicate that the embodiment(s)or example(s) so described may include a particular feature, structure,characteristic, property, element, or limitation, but that not everyembodiment or example necessarily includes that particular feature,structure, characteristic, property, element or limitation. Furthermore,repeated use of the phrase “in the preferred embodiment” does notnecessarily refer to the same embodiment, though it may.

What is claimed is:
 1. A cascode power amplifier (PA) comprising: acommon gate transistor with a gate connected to an biasing transmissionline, a source connected to an output line and a drain; a common sourcetransistor with a to connected to an RF input that receives an RF inputsignal, a source connected to the drain of the common gate transistor,and a drain connected to a feedback line; a first resistor between thebiasing transmission line and the source of the common sourcetransistor; a second resistor between the biasing transmission line andthe feedback line, wherein the first transistor and the secondtransistor split a drain voltage of the common source transistor equallybetween the common gate transistor and the common source transistor; athird resistor and a first capacitor connected in series between thebiasing transmission One and the feedback line; and a second capacitorconnected in series between the biasing transmission line and feedbackline configured to short an RF signal to ground.
 2. The cascode PA ofclaim 1 where the values of the first resistor and the second resistorare between 4000 Ohms and 6000 Ohms, the value of the third resistor isbetween 250 and 400 Ohms, the value of the first capacitor is between0.5 Pico farads (pF) and 1.5 pF, and where the value of the secondcapacitor is between 0.3 pF and 1A pF.
 3. The cascade PA of claim 1wherein the source of the common gate transistor is connected to ground.4. The cascode PA of claim 1 wherein the biasing transmission line haswidth between 6 micro meters (um) and 14 um and has a length between 300um and 500 um.
 5. The cascade PA of claim 1 wherein the feedback line isa transmission line between the first resistor and the drain of thecommon source transistor that has a length that is between 3 and 6 timesits length.